The present invention relates in general to electronic circuits and, more particularly, to logic circuits.
Many logic family applications have logic devices that operate within a mixed signal environment. The logic devices have logic drivers that may communicate with a logic receiver of a different logic family type. Typically, different logic family devices communicate with each other using translators to convert, for example, an ECL signal from the logic driver to a CMOS signal received at the logic receiver. A different type of translator is required for each type of logic driver and logic receiver used within the mixed signal environment. In addition, systems usually have an external termination scheme on an interconnect transmission line between the logic driver and logic receiver so the logic receiver circuit is terminated to receive the specific logic driver family type. The termination is a resistance that provides a termination for the logic device through to a voltage source Vtt. The voltage source Vtt is typically different for each logic family application. The resistance is typically chosen to equal the impedance of the interconnect transmission line to help reduce interconnect signal distortion. It is more of an advantage to have terminations as close as possible to the logic receiver circuit to help reduce interconnect signal distortion even more. Also, prior art termination schemes typically require different termination connections are used for each type of logic family device. For example, to use an ECL logic device requires a 50 ohm termination to a Vtt voltage source. A CML logic device may require termination through a resistance to a different voltage source. Most prior art logic family devices also have the termination resistors hard-wired to a circuit board making it difficult to change terminations for different logic family applications.
Hence, it is desired to have a logic receiver circuit that is programmable to allow the logic receiver circuit to communicate with different logic family driver circuits. Furthermore, it is desirable to have the terminations internal to the logic receiver circuit package so the terminations are close to the receiving circuit to help eliminate transmitted signal noise. The invention disclosed herein will address the above problems.